1. Field of the Invention
The present invention relates generally to diode pumped laser apparatus, and more particularly to an improved intra-cavity tripled diode pumped Nd:YVO4 laser utilizing an angle-tuned LBO doubler and tripler and operated at high repetition rates and good overall efficiency.
2. Brief Description of the Prior Art
Laser radiation at 355 nm with high average power and high repetition rate (above 10 kHz) is useful for applications such as stereo lithography in which a UV-sensitized liquid polymer is laser-scanned and solidified to form solid models of complicated computer-generated mechanical parts in a layer-by-layer fashion. Third harmonic generation of the nominally 1 micron output from Nd:Host materials operated in high repetition rate Q-switched cavities have been obtained in the prior art using extra-cavity architectures such as that illustrated in FIG. 1 of the drawing. As schematically shown at 10, such a laser typically consists of a pair of cavity mirrors M1 and M2, some sort of diode-pumped gain medium S, and an intra-cavity Q-switch Q. The output of the laser operating at a wave length of 1064 nm passes from M1 out to a suitable lensing means L1 and is focused into a doubler D which generates a second harmonic at 532 nm along with the remaining fundamental at 1064 nm. The fundamental and second harmonic are then focused by a second lens means L2 into a tripler T where a third harmonic radiation is generated at 355 nm. Appropriate means not shown but well known in the art is then used to separate out the third harmonic radiation from the fundamental and second harmonic. Relatively narrow fundamental pulse widths (10-20 ns) and non-critically phase-matched (NCPM) doubling allows good conversion efficiencies to be obtained with Lithium triborate (LBO) as an extra-cavity type I doubler and type II sum-frequency tripler. However, operation of the NCPM doubler at the phase match temperature of 150.degree. C. may complicate the optical and mechanical design in systems which require close spacing between a doubler and tripler with widely different temperatures. It is therefore believed that certain advantages and improvements could be obtained with an totally intra-cavity architecture. Whereas diode pumped lasers are often doubled and tripled extra-cavity, intra-cavity doubling/tripling has been demonstrated with arc lamp pumping. In addition, others have performed intra-cavity doubling followed by extra-cavity tripling. However, it has been shown that angle-tuned LBO may be used at room temperature for efficient intra-cavity second harmonic and third harmonic generation in a flash lamp pumped Nd:YAG cavity at relatively low repetition rates (below 1 kHz). The high fundamental intra-cavity peak power achieved with the lamp pumping permits the use of relatively simple cavities with few optical components and slowly varying spot sizes. We are not aware of any diode pumped solid state lasers using intra-cavity doubling and tripling within a single cavity.
In an intra-cavity architecture, the fundamental beam and UV (tripled) beam are co-linear and must be separated. Previously, this has been done by using mirrors with special coatings that imperfectly transmit the UV while reflecting the intra-cavity fundamental. Improvements must thus be made in separating the UV beam from the fundamental. In intra-cavity doubled standing wave multi-mode lasers, two second harmonic beams are produced in a doubler, and only one of these beams is readily available for further use. Although a mirror can be used to recycle one of these beams to overlap the first, dispersion in air can introduce a phase shift between the beams, reducing the effective power of the overlapping beams. Means must therefore be provided for dealing with this phase shift problem.
Acousto-optic Q-switches operated at high power are usually water-cooled and, if used intra-cavity, would require that flexible cooling lines extend into the cavity to allow the necessary rotation and translation of the switch for correct optical alignment. An intra-cavity architecture therefore would require an alternative means of cooling the Q-switch if it is desired to remove water cooling tubing from within the laser cavity enclosure.
In order to improve the pumping efficiency in a side-pumped solid state laser, pump mode and cavity mode overlap can be achieved by a shallow bounce off of a pumped face. However, efficient operation depends upon high pump absorption coefficient and shallow bounce angle. Therefore, a long collimated pumping source placed tightly against the pumped face has been used with the result that diffraction losses (beam clipping) occurs at the ends of the slap unless the slab is considerably longer than the pumping length. In addition, side-pumped slabs and rods often yield poor laser mode profiles and poor efficiency because of non-uniform thermal lensing induced by asymmetric pumping profiles and non-uniform thermal boundaries. Previous solutions to this problem have included the use of cylindrical lenses to provide elliptical cavity modes to compensate for asymmetric thermal lensing. The provision of an intra-cavity architecture therefore requires consideration of multiple design considerations.